No abstract

The purpose of this study was to investigate the frequency and impact of vertical mis‐centering on organ doses in computed tomography (CT) exams and evaluate the effect of a commercially available positioning compensation system (PCS). Mis‐centering frequency and magnitude was retrospectively measured in 300 patients examined with chest‐abdomen‐pelvis CT. Organ doses were measured in three postmortem subjects scanned on a CT scanner at nine different vertical table positions (maximum shift ± 4 cm). Organ doses were measured with optically stimulated luminescent dosimeters inserted within organs. Regression analysis was performed to determine the correlation between organ doses and mis‐centering. Methods were repeated using a PCS that automatically detects the table offset to adjust tube current output accordingly. Clinical mis‐centering was >1 cm in 53% and 21% of patients in the vertical and lateral directions, respectively. The 1‐cm table shifts resulted in organ dose differences up to 8%, while 4‐cm shifts resulted in organ dose differences up to 35%. Organ doses increased linearly with superior table shifts for the lung, colon, uterus, ovaries, and skin (R2 = 0.73–0.99, P < 0.005). When the PCS was utilized, organ doses decreased with superior table shifts and dose differences were lower (average 5%, maximum 18%) than scans performed without PCS (average 9%, maximum 35%) at all table shifts. Mis‐centering occurs frequently in the clinic and has a significant effect on patient dose. While accurate patient positioning remains important for maintaining optimal imaging conditions, a PCS has been shown to reduce the effects of patient mis‐centering.

This study assesses the accuracy of effective atomic number (Z eff ) and electron density measurements acquired from dual energy CT and characterizes the response to clinically relevant variables representative of challenges in patient imaging, including: phantom size, material position within the phantom, variation over time, off-center positioning, and large cone beam angle. Methods: The Gammex Multi-Energy CT head and body phantoms were used to measure Z eff and electron density from 35 rod inserts that mimic tissues and varying concentrations of iodine and calcium. Scans were performed on a Canon Aquilion ONE Genesis CT scanner over a period of 6 months using default dual energy protocols appropriate for each phantom size. Theoretical Z eff and electron density values were calculated using data provided by the phantom manufacturer and compared to the measurements. Sources of variance were separated and quantified to identify the influences of random photon statistics, ROI placement, and variation over time. A subset of measurements were repeated with the phantom shifted in the vertical and horizontal directions, and over all slices in the volumetric scan. Results: All measurements showed strong correlation (r > 0.98) with their corresponding theoretical values; however, the system did demonstrate a bias of −0.58 atomic units in the body phantom and 0.28 atomic units in the head phantom for Z eff measurements. The mean absolute percent error (MAPE) was 6.3% for the body phantom and 3.2% for the head phantom. Electron density measurements of the body and head phantoms gave MAPE values of 4.6% and 1.0%, respectively. Z eff and electron density measurements significantly varied within the solid water background, showing a positional dependence within the phantom that dominated the total standard deviation in measurements. Z eff values dropped by 0.2 atomic units when the phantom was off-center; electron density measurements were less affected by phantom position. Along the z-axis, the accuracy drops off markedly at more than 50-60 mm from the central slice. Conclusion:The Canon dual energy system offers an accurate way of measuring the Z eff and electron density of clinically relevant materials. Accuracy could be improved further by calibration to remove bias, careful attention to centering within the FOV, and avoiding measurements at the edges of the cone beam.

Purpose:We compare the effect of tube current modulation (TCM) and fixed tube current (FTC) on size-specific dose estimates (SSDE) and image quality in lung cancer screening with low-dose CT (LDCT) for patients of all sizes.Methods: Initially, 107 lung screening examinations were performed using FTC, which satisfied the Centers for Medicare & Medicaid Services' volumetric CT dose index (CTDI vol ) limit of 3.0 mGy for standard-sized patients. Following protocol modification, 287 examinations were performed using TCM. Patient size and examination parameters were collected and waterequivalent diameter (D w ) and SSDE were determined for each patient. Regression models were used to correlate CTDI vol and SSDE with D w . Objective and subjective image quality were measured in 20 patients who had consecutive annual screenings with both FTC and TCM.Results: CTDI vol was 2.3 mGy for all FTC scans and increased exponentially with D w (range = 0.96-4.50 mGy, R 2 = 0.73) for TCM scans. As patient D w increased, SSDE decreased for FTC examinations (R 2 = 1) and increased for TCM examinations (R 2 = 0.54). Image quality measurements were superior with FTC for smaller sized patients and with TCM for larger sized patients (R 2 > 0.5, P < 0.005). Radiologist graded all images acceptable for diagnostic evaluation of lung cancer screening.Conclusion: Although FTC protocol offered a consistently low CTDI vol for all patients, it yielded unnecessarily high SSDE for small patients and increased image noise for large patients. Lung cancer screening with LDCT using TCM produces radiation doses that are appropriately reduced for small patients and increased for large patients with diagnostic image quality for all patients. K E Y W O R D S image quality, lung cancer screening CT, radiation dose, size-specific dose estimates, tube current modulation ---